The optical networks in the future are required, for example, to be formed over a conventional optical communication system, and to allow processing for inserting, dropping, and switching control light in devices (such as optical repeater nodes) remote from terminal stations. At this time, from the viewpoint of the efficient energy use, it is effective to minimize the number of conversions between optical signals and electric signals during propagation and processing of information.
In the currently used optical repeater nodes and the like, signals are processed by using conversions between optical signals and electric signals as in the terminal stations. For example, control light transmitted to an optical repeater node or the like is once converted into an electric signal, is then electrically processed, and is thereafter converted into an optical signal. Therefore, the equipment construction is complex, and great electric power is needed for compensating for loss caused by the conversions between optical signals and electric signals.
In addition, conventionally, a repeater station which uses phase-conjugate light, is arranged between a transmitter station and a receiver station, and is connected to the transmitter station and the receiver station through an optical transmission line is disclosed, for example, in Japanese Registered Patent No. 3436310. The repeater station contains a phase-conjugate-light generating device and a modulation means. The phase-conjugate-light generating device contains a control-light/excitation-light supply means and a control-light/phase-conjugate-light extraction means. The control-light/excitation-light supply means supplies to a nonlinear optical medium excitation light and inputted control light which is transmitted from the transmitter station, and the control-light/phase-conjugate-light extraction means extracts output control light and phase-conjugate light which are generated by the nonlinear optical medium in response to the inputted control light and the excitation light. The modulation means modulates the excitation light with monitor data unique to the repeater station. The repeater station transmits the phase-conjugate light to the receiver station, where the phase-conjugate light contains the modulated monitor data.
However, in the case where data is inserted into and transmitted through the conventional optical networks, data are collected in a terminal station or optical repeater nodes arranged at a plurality of specific positions, and are then respectively transmitted by use of dedicated light waves.